One of the conventional wireless communications systems using the FDD architecture is a wireless communications system using a DSRC (Dedicated Short Range Communications) architecture (hereinafter referred to as a “DSRC system”).
The DSRC system standard specifies that when transmitting a first wireless signal from a first wireless communications device provided on the road (hereinafter referred to as a “base station”) to a second wireless communications device provided in a vehicle (hereinafter referred to as a “mobile station”) (hereinafter such a transmission will be referred to as a “downlink”), one of 5775 [MHz], 5780 [MHz], 5785 [MHz], 5790 [MHz], 5795 [MHz], 5800 [MHz] and 5805 [MHz] is used as the center frequency.
The DSRC system standard also specifies that when transmitting a second wireless signal from the mobile station to the base station (hereinafter such a transmission will be referred to as an “uplink”), a center frequency that is away from that used for the downlink by 40.000 [MHz] is used. Specifically, if 5775 [MHz] is used as the center frequency for the downlink, 5815 [MHz] is used as the center frequency for the uplink. Similarly, if 5780 [MHz] is used for the downlink, 5820 [MHz] is used for the uplink. If 5785 [MHz] is used for the downlink, 5825 [MHz] is used for the uplink. If 5790 [MHz] is used for the downlink, 5830 [MHz] is used for the up link. If 5795 [MHz] is used for the downlink, 5835 [MHz] is used for the uplink. If 5800 [MHz] is used for the downlink, 5840 [MHz] is used for the uplink. If 5805 [MHz] is used for the downlink, 5845 [MHz] is used for the uplink.
In a section on the technical requirements for wireless equipment in DSRC system standard specifications, standards for image response are specified only for the base station.
Where a demodulation process is performed by a digital signal processing circuit, in order to convert a received modulated high-frequency signal to a frequency suitable for the digital signal processing circuit, a frequency conversion circuit for downconverting the modulated high-frequency signal needs to be provided preceding the digital signal processing circuit.
In view of the technical requirements for wireless equipment, it is preferred that a frequency conversion circuit employing a LOW-IF architecture, for example, is used in the base station. This is because it is possible with the LOW-IF architecture to remove an image disturbing signal without using an image suppression filter in a high-frequency part, as described in Non-Patent Document 1 (J. Crols and Michiel S. J. Steyaert, “Low-IF Topologies for High-Performance Analog Front Ends of Fully Integrated Receivers”, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS II: ANALOG AND DIGITAL SIGNAL PROCESSING, VOL. 45, NO. 3, March 1998).
As described in Non-Patent Document 1, with the LOW-IF architecture, the center frequency of a received modulated high-frequency signal is downconverted to a frequency that is about a few times as great as the signal bandwidth of the modulated high-frequency signal. Then, the downconverted signal is directly sampled by a sampler and demodulated by a digital signal processing circuit. The LOW-IF architecture is advantageous in that it offers better reception characteristics and high degrees of integration.
For the mobile station, however, no image response standard is specified. Therefore, it is possible to use a frequency converter in which a local oscillator is shared by the transmitter and the receiver. Thus, with the mobile station, a single-conversion architecture can be employed. Therefore, the mobile station can be provided at a low cost.
As described above, where a frequency converter employing the LOW-IF architecture is used in the base station, a frequency-converted signal is converted into a signal having a frequency that is about a few times as great as the signal bandwidth of the received modulated high-frequency signal.
Where the mobile station uses a frequency converter employing the single-conversion architecture in which a local oscillator is shared by the transmitter and the receiver, a frequency-converted signal is converted into a signal having a frequency that is equal to the difference between the uplink and the downlink. Typically, the frequencies are different from each other. This is illustrated in FIG. 20 to FIG. 22.
FIG. 20 is a diagram schematically showing a conventional base station 9000 and a conventional mobile station 9001 communicating with each using the DSRC system. In FIG. 20, a frequency fc denotes the center frequency for the uplink, and the value thereof is one of 5815 [MHz], 5820 [MHz], 5825 [MHz], 5830 [MHz], 5835 [MHz], 5840 [MHz] and 5845 [MHz]. Moreover, in FIG. 20, a frequency fd denotes the difference between the center frequency of the signal used for the uplink and that of the signal used for the downlink, and the value thereof is 40.000 [MHz]. As shown in FIG. 20, a signal is uplinked from the mobile station 9001 to the base station 9000 with the center frequency fc. A signal is downlinked from the base station 9000 to the mobile station 9001 with a center frequency fc−fd. In the DSRC system, it is specified that the channel bandwidth is 5 [MHz].
FIG. 21 is a diagram showing a general configuration of a conventional base-station wireless communications device employing the LOW-IF architecture. FIG. 22 is a diagram showing a general configuration of a conventional mobile-station wireless communications device employing the single-conversion architecture. For the purpose of simplifying the problem, the following description will only discuss the signal-receiving operation at the mobile-station wireless communications device and the base-station wireless communications device.
First, referring to FIG. 20 and FIG. 21, the signal-receiving operation at the base-station wireless communications device will be described. In FIG. 21, the base-station wireless communications device includes an antenna 9200, a band-pass filter 9216, a transmission/reception selector switch 9211, an amplifier 9201, a first mixer 9202, a second mixer 9203, a first local oscillator 9206, a first low-pass filter 9204, a second low-pass filter 9205, a first sampler 9207, a second sampler 9208, a sampling signal generator 9209, a demodulation digital circuit 9210, a transmission high-frequency circuit 9212, a third mixer 9213, a second local oscillator 9214 and a transmitter circuit 9215.
In the base-station wireless communications device, the signal-receiving operation is performed by using the antenna 9200, the band-pass filter 9216, the transmission/reception selector switch 9211, the amplifier 9201, the first mixer 9202, the second mixer 9203, the first local oscillator 9206, the first low-pass filter 9204, the second low-pass filter 9205, the first sampler 9207, the second sampler 9208, the sampling signal generator 9209 and the demodulation digital circuit 9210.
In the signal-receiving operation, the transmission/reception selector switch 9211 is switched so that the antenna 9200 and the amplifier 9201 are connected to each other. A modulated high-frequency signal R(t) from the mobile station 9001 received by the antenna 9200 whose center frequency is fc is inputted to the amplifier 9201. The amplifier 9201 amplifies the modulated high-frequency signal R(t) to an appropriate level, and inputs the amplified signal to the first mixer 9202 and the second mixer 9203. The first local oscillator 9206 outputs a sine wave whose center frequency is fc−fa. As described in Non-Patent Document 1, it is preferred that fa is a frequency that is about a few times as great as the channel bandwidth of the modulated high-frequency signal R(t).
The first mixer 9202 multiplies the sine wave outputted from the first local oscillator 9206 whose center frequency is fc−fa with the modulated high frequency signal R(t) to output a modulated low-to-intermediate-frequency signal in-phase component RXI(t) whose center frequency is fa.
The second mixer 9203 multiplies a signal outputted from the first local oscillator 9206 whose center frequency is fc−fa and whose phase is shifted from that of the sine wave by π/2 with the modulated high-frequency signal R(t) to output a modulated low-to-intermediate-frequency signal quadrature component RXQ(t) whose center frequency is fa.
The first sampler 9207 samples the modulated low-to-intermediate-frequency signal in-phase component RXI(t) in synchronism with a signal outputted from the sampling signal generator 9209 whose frequency is fs1 to output an in-phase component sampled signal I(mTs1).
The second sampler 9208 samples the modulated low-to-intermediate-frequency signal quadrature component RXQ(t) in synchronism with a signal outputted from the sampling signal generator 9209 whose frequency is fs1 to output a quadrature component sampled signal Q(mTs1).
Herein, m is an integer, and Ts1 is the inverse of the sampling signal frequency fs1, .e., Ts1=1/fs1. In order to facilitate the signal processing operation at the demodulation digital circuit 9210, fs1 is in many cases set to a value that is equal to fa multiplied by 2N (N is a natural number: N=1, 2, 3, . . . ).
The demodulation digital circuit 9210 receives the in-phase component sampled signal I(mTs1) and the quadrature component sampled signal Q(mTs1) as input signals, and demodulates the signals to output received data after removing the image disturbing signal, as described in Non-Patent Document 1.
Next, referring to FIG. 21 and FIG. 22, the signal-receiving operation at the mobile-station wireless communications device will be described. In FIG. 22, the mobile-station wireless communications device includes an antenna 9100, a band-pass filter 9112, a transmission/reception selector switch 9108, an amplifier 9101, a first mixer 9102, a local oscillator 9103, a low-pass filter 9104, a sampler 9105, a sampling signal generator 9106, a demodulation digital circuit 9107, a transmission high-frequency circuit 9109, a second mixer 9110 and a transmitter circuit 9111.
In the mobile-station wireless communications device, the signal-receiving operation is performed by using the antenna 9100, the band-pass filter 9112, the transmission/reception selector switch 9108, the amplifier 9101, the first mixer 9102, the local oscillator 9103, the low-pass filter 9104, the sampler 9105, the sampling signal generator 9106 and the demodulation digital circuit 9107.
In the signal-receiving operation, the transmission/reception selector switch 9108 is switched so that the antenna 9100 and the amplifier 9101 are connected to each other. A modulated high-frequency signal RL(t) from the base station 9000 received by the antenna 9100 whose center frequency is fc−fd is first passed through the band pass filter 9112 to remove signals of frequency bands that are used neither in the base station nor in the mobile station, and is then inputted to the amplifier 9101. The amplifier 9101 amplifies the modulated high-frequency signal RL(t) to an appropriate level, and inputs the amplified signal to the first mixer 9102. The first local oscillator 9103 outputs a sine wave whose center frequency is fc.
The first mixer 9102 multiplies the sine wave outputted from the local oscillator 9103 whose center frequency is fc with the modulated high-frequency signal RL(t) to output a modulated low-to-intermediate-frequency signal L(t) whose center frequency is fd to the low-pass filter 9104.
In the frequency conversion at the first mixer 9102, a signal whose center frequency is fc+fd is an image disturbing signal. However, since the image response is not specified in the technical requirements for wireless equipment used in the mobile station in the DSRC system standard, a lower-order, inexpensive low-pass filter can be used as the filter following the first mixer 9102. If the image disturbing signal were to be a problem, signal components of only the necessary bands can be extracted by using a complex filter.
The sampler 9105 samples the modulated low-to-intermediate-frequency signal L(t) outputted from the low-pass filter 9104 whose center frequency is fd in synchronism with a signal outputted from the sampling signal generator 9106 whose frequency is fs2 output a sampled signal Ls(mTs2). Herein, m is an integer, and Ts2 is a value represented by the inverse (1/fs2) of the sampling signal frequency fs2. In order to facilitate the signal processing operation at the demodulation digital circuit 9107, fs2 is in many cases set to a value that is equal to fd multiplied by 2N (N is a natural number: N=1, 2, 3, . . . ).
The demodulation digital circuit 9107 receives the sampled signal Ls(mTs2) as an input signal, and demodulates the signal to output received data.
Other background art publications related to the present invention include Non-Patent Document 2 (Mikko Valkama, et al., “Advanced Methods for I/Q Imbalance Compensation in Communication Receivers” IEEE TRANSACTIONS ON SIGNAL PROCESSING, Vol. 49, No. 10, pp. 2335-2344, October 2001) and Non-Patent Document 3 (Kiyomichi Araki ed., “Software Musen No Kiso To Oyo (Basics and Applications of Software Radio)”, SIPEC Corporation Knowledge Service Department, p. 123, October 2002).
As described above, in the base station employing the LOW-IF architecture, the center frequency fa of the signals RXI(t) and RXQ(t) inputted to the first and second samplers 9207 and 9208 is about a few times as great as the signal bandwidth of the modulated high-frequency signal R(t). In the mobile station employing the single-conversion architecture, the center frequency fd of the signal L(t) inputted to the sampler 9105 is equal to the difference (40.000 [MHz]) between the uplink frequency and the downlink frequency as specified in the DSRC system standard.
Therefore, the center frequency of the signal inputted to the sampler in the mobile station is substantially different from that of the signal inputted to the sampler in the base station, whereby the frequency of the sampling signal used in the sampler 9105 in the mobile station is different from that of the sampling signal used in the first and second samplers 9207 and 9208 in the base station.
Therefore, with the conventional system, the sampling frequency for the base station and that for the mobile station need to be set to different values even though their demodulation digital circuits are substantially the same in function. Thus, it is necessary to provide two different demodulation digital circuits for the base station and for the mobile station. Although it is desirable to realize a common demodulation digital circuit for the base station and for the mobile station in order to provide an inexpensive transceiver, it is difficult to realize such a common demodulation digital circuit for reasons stated above.